Data center design standards for cabinet and floor loading

The 800-pound gorilla in the room has been on a binge and can no longer be ignored. In fact, it has now become an elephant, weighing in at 2,500-3,000 pounds. We’re talking about today’s data center equipment cabinets, which can be heavier than many floors can support. Let’s look at the implications of this added rack and cabinet weight and consider some data center design tactics for managing the bulk.

Addressing equipment weight changes and issues
Why does data center equipment weigh so much more now than it did in the past? A decade ago, we still stacked six to eight desktop computers on shelves in a standard rack or cabinet, but those machines were mostly air inside. Today, the boxes are built with far more hardware inside. Miniaturized components have let us squeeze more devices into a much smaller chassis, resulting in thinner hardware (think 1U, 2U, and blades). That means we can put much more hardware in a cabinet to make better use of floor space. But that space efficiency comes with some penalties.

Cooling has been the most recognized concern, because the higher density of heat-producing electronics requires more aggressive cooling tactics. However, the challenging problem of cooling often causes people to overlook the increased weight of IT equipment and its impact on the structure. When each server weighs more, and we pack more of them together, the racks and cabinets have to be physically larger and stronger. The result is that most quality cabinets today can handle 2,500-3,000 pounds. That’s as much as many uninterruptable power supply (UPS) and battery cabinets weigh, which used to be the only things in the data center that caused us to worry about weight.

What this means to data center design and the building structure is that either the floor slab has to be stronger than what is found in most buildings, or we have to spread the load across more of the floor to keep it from sagging or falling through. We might be able to beef-up the floor slab by adding steel beams and/or columns underneath, but that’s a major undertaking that is often not possible or realistic. Or we could put a structure on top of the floor that spreads the cabinet load across more floor area.  A properly designed raised access floor might do this, or a low platform of steel beams might be used, laid across the floor slab with a false floor on top of them. (This is known as “dunnage," and has been used for years to distribute things like UPS weights and big equipment on roofs.) Of course we could also spread the load over a larger floor area by using more cabinets and only partially filling them, or by increasing aisle widths so the cabinets rows are spaced farther apart. These last two options waste precious space, but they could be easier and less expensive than structural changes. What wecan’t do is just ignore the issue and assume the floor will hold, because it may not. If it doesn’t, you’ll be faced with a live data center in danger of collapse, and responding to that kind of emergency is almost always disruptive, because the equipment has to be moved fast!

Architects are always shocked when we give them floor loading requirements for a modern data center, which can be in the same category as library book stacks or concentrated file cabinets. The data center design process for a new building is difficult and costly enough, but dealing with the structure in an existing building is even more challenging. Strengthening a floor from below may not be practical. The lower building structure may be unable to support more weight above, or there may be a tenant below your floor that can’t be disrupted by the messy job of adding steel beams. The issue has become so important that the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) published a book called Structural and Vibration Guidelines for Datacom Equipment Centers to address the subject in much greater detail.

Floor loading examples
We used to figure cabinets at about 1,000 pounds each, and they were generally 24 inches wide by 36 inches deep. That’s about 167 pounds per square foot (PSF), which is still above the weight limits of most office buildings. But in reality, that weight is spread over half the aisle in front and in back of the cabinets, which is known as the “cabinet footprint." So the average maximum floor loading from those cabinets was actually more like 70-80 PSF, depending on aisle width. Most office floors can support that.

If you increase the cabinets to 1,500 pounds, the loading becomes 100-125 PSF. Now we’re out of “normal” office building structure range, but you can still find buildings with special floors or floor areas that are designed to handle that kind of weight. On the other hand, cabinets weighing 3,000 pounds and arranged with 36-inch aisles drive the floor loading to250 PSF. You may find that capacity on the ground floor of a warehouse, but generally this requires special design and construction.

“Low profile” 1U servers are also much deeper, so we gain a little weight distribution benefit (a lower PSF), because they won’t fit in 36-inch deep cabinets. Today, we tend to use cabinets that are 42 inches deep or more. And since we need more space just to install the deeper equipment in the deeper cabinets, 48-inch aisle widths are now more common as well. The deeper cabinets and increased aisle widths result in greater square footage per cabinet footprint, which spreads the load a little more, but it still doesn’t compensate for the really heavy equipment. If cabinets remain at only 24 inches wide, we can still have floor loading problems.

A good data center design practice today, for reasons other than structure, is to increase cabinet widths to 30 inches. This provides space for all the power cords and cables that can pile up and block airflow behind equipment in narrower cabinets. It also increases the cabinet footprint and helps spread the load a little more. Wider, deeper cabinets can enable us to support 1,500-pound cabinets on standard office floors, and 2,000-pound cabinets on the stronger floors found in some buildings, but beyond that we still need structural help.

Some operations have chosen abnormally tall cabinets that save floor space (48U-52U instead of the standard 42U). These tall cabinets can hold more equipment, but they invite even greater weight concerns. Today we ask for at least 150 PSF floors in new data centers. And if we anticipate full cabinets, narrow cabinets and/or tall cabinets, we may even prefer 250 PSF capacity. All require special floor structures.

Floor loading can actually be one of the deciding factors in the “raised floor vs. no raised floor” debate. As noted above, a good, well-interlocked, bolted stringer floor system can help spread the cabinet load over the floor area, letting you distribute your cabinet loads over more of the slab. Of course, the raised access floor has to be strong enough to support the cabinets, and the raised floor structure and panels add weight themselves. Some people advocate cast aluminum raised floor panels to achieve maximum floor loading capacity with minimum weight, but cost can then become a significant factor. 

If you’re wondering exactly how much it would cost per square foot to design a steel-framed building with floors of different capacities, here is a guideline based on New York costs: Increasing 100-125 lbs/s.f. is $2 per s.f. and up, 150-175 lbs/s.f. is $10 per s.f. and up, and increasing to 250 lbs/s.f. is $25 per s.f and up. Of course, these numbers vary for poured concrete, and they do not address what it would cost to upgrade an existing floor.

In short, the a data center is a “system," and the entire data center design needs to be taken into account. Power and heat estimates are certainly important, but you can’t underestimate cabinet weights either, and only a qualified structural engineer should advise on real structural issues.

About the author: Robert McFarlane is Principal in charge of data center design for the international consulting firm Shen Milsom &Wilke LLC. McFarlane has spent more than 35 years in communications consulting, has experience in every segment of the data center industry and was a pioneer in developing the field of building cable design. McFarlane also teaches the data center facilities course in the Marist College Institute for Data Center Professional  program, is a data center power and cooling expert, is widely published, speaks at many industry seminars and is a corresponding member of ASHRAE TC9.9 which publishes a wide range of industry guidelines.